The present invention relates to electro-optical scanning devices, particularly those devices used for reading any two dimensional printed surface for the purpose of making a duplicate or digital record of the information contained on the surface.
In one type of electro-optical reader, the subject surface is illuminated by scanning with a focused laser beam and the light reflected off the surface is collected to obtain a representation of that surface.
In optical scanning systems it would be economically and practically desirable to use a single optical system to illuminate and read the document. However, in the past, certain system incompatibilities have prevented this type of design. Because the majority of surfaces of documents to be examined are opaque, such an optical reader must be designed to extract its information from light reflected off the opaque surface.
Light reflected off any surface may be characterized as having two components: both a specular and a diffuse component. In the case of a paper document, if the paper and ink have similar specular reflectances, the specular reflection off the surface will be devoid of information, while the diffuse component will carry all the meaningful information to be extracted. Where the axis of incident illumination is normal to the paper surface, the specular reflection will also be centered about that axis. Therefore, to increase the collection of the information-bearing or diffuse component, the optical axis of the reader detector is placed at an angle from twenty (20) degrees to fifty (50) degrees from the paper normal. Since the specular reflection, like glare, is generally much brighter than the diffuse component, the diffuse component must be collected from the entire annulus surrounding the specular component to improve the signal-to-noise ratio.
In the past, optical scanning readers have collected the diffuse component from a portion of the desired collection annulus by placing two or more linear arrays of optical fibers parallel to the scan line at an angle of forty-five (45) degrees from the paper surface, such that each array cuts through the desired collection annulus. These fibers were bundled to terminate in a single detector so that each illuminated position on the document surface could be related to a time varying detector signal. Systems which employ this type of fiber optic assembly are generally very expensive, because of the cost of the fiber optic assembly itself, and because they require a large area detector, for example, a photomultiplier, to measure the collected light. A further disadvantage of these systems is that their efficiency is limited by the number of fiber arrays which can physically be placed in the annulus, and by the size of the photomultiplier or detection surface which terminates the fiber bundle.
It is accordingly a primary object of the present invention to provide an improved apparatus for the reading and/or writing of a surface.